The invention is applicable to any oven and, in particular, to a strip floater oven which is used in conjunction with a device for applying some type of coating, e.g. paint, to a continuous element such as a sheet of metal. Such an oven is described in my copending application Ser. No. 046,796 filed June 8, 1979 now U.S. Pat. No. 4,299,036 and generally comprises a number of horizontally aligned chambers which are disposed side by side and sealed from each other and the ambient atmosphere. A sheet of metal is guided horizontally through the coating device and then successively through the individual heat treatment chambers or zones wherein it is contacted with heated gas to dry and cure the coating of paint by removal of the paint carrying solvent as a highly volatile vapor in the heated gas exhausted from the various chambers.
Heated gas is impinged upon the traveling sheet of metal in each of the heat treatment chambers from a number of nozzles which are positioned vertically above and below the sheet of metal and which are normally at least coextensive with the width of the sheet of metal. Good workable nozzles for use in so-called HV (high velocity) zones where gases are impinged upon the traveling element or web at relatively high velocities are of the floatation type, as disclosed in U.S. Pat. Nos. 3,837,551 and 3,982,327. A nozzle of the direct impingement type, as disclosed in U.S. Pat. No. 2,574,083, is used in so-called PH (preheat) zones wherein the element is preheated, prior to passage through the HV zones.
In prior art ovens, heated gas is brought to and exhausted from, the individual chambers of the oven in much the manner taught, for example, by U.S. Pat. No. 3,923,449. Generally, each treatment chamber is provided with a costly burner system which is used to individually condition the temperature of the heated gas circulated to that particular chamber. Spent gas, including solvent vapor, is separately removed from each of the chambers and exhausted in a common flue. The volume of gas exhausted from the oven is predetermined to maintain the concentration of solvent vapor at or below 25% of its lower explosive limit (LEL). Higher solvent vapor concentration (up to 50% of the LEL) may be used if the solvent vapor concentration of the exhaust gas is carefully monitored. This is done by continuously removing a portion of the exhaust gas from the common flue and measuring it for its solvent content. It can be appreciated that the solvent content of gas in the main exhaust stream is not a true reflection of the actual concentration of solvent vapor in any of the individual chambers or zones. For example, the concentration of solvent vapor may be dangerously high in one chamber, but offset by a low concentration of solvent vapor in another chamber.
The oven of the invention of my aforementioned copending application utilizes a single source of heat, such as a conventional fume incinerator, as a means of heating gas which is successively cascaded through the various chambers from the last to the first chamber to be encountered by the traveling element, such that a measure of the solvent vapor concentration of the exhaust gas exiting the first to be encountered chamber, is a true representation of the maximum concentration of accumulated solvent vapor within the oven.
The oven of this invention employs a central heat source, but it is utilized differently from that described in my copending application. Further, this invention is directed to what is believed to be an even simpler system for controlling the temperature of the heated gas in the various chambers or zones of a multi-zone oven.
Briefly stated, the invention is in an oven which comprises at least one chamber which is substantially sealed from the ambient atmosphere and which is designed to have a web or element, to be heated, passed substantially horizontally therethrough. Means are provided for circulating gas, heated at a single outside heat source to a certain temperature, to the chamber at a predetermined constant pressure for subsequent impingement against the element at a relatively high velocity as the element travels through the chamber. Means are supplied for mixing cooler gas with the heated gas, prior to circulation of the mixture to the chamber to produce, in the chamber, a predetermined desired gas temperature which is monitored by a temperature sensing device that acts to regulate the amount of hot and cool gas in the mixture. Means are provided for exhausting gas from the chamber at a predetermined flow rate. Means are used for monitoring the gas pressure within the chamber and for changing the amount of cooler gas in the mixture, when the gas pressure within the chamber varies from a desired norm, after the oven is, in balance, and operating at a desired temperature. The change of cooler gas in the mixture naturally influences the gas temperature being monitored in the chamber by the temperature sensing means to correspondingly regulate the mixture of hot and cool gas to return the chamber back to the desired temperature. The balance of the oven is controlled and maintained primarily by modulating the flow of cool gas while keeping the pressure of the high temperature gas constant.
Other aspects of the invention are the use of a fume incinerator as a source of heating the gas to high temperatures, and the circulation of the exhaust gas to the fume incinerator for reheating and recirculation to the chamber. Also, gas is constantly removed from the chamber for combination with the mixture of hot and cool gas, prior to circulation of the mixture to the chamber.
The basic oven of this invention is highly simplified and does not require the use of a number of costly gas burners as do prior art ovens, or the use of heat exchangers as does the oven of my copending application, since the gas, heated in the single fume incinerator, is circulated directly to the various chambers or zones. The invention is readily adapted to existing ovens because of its simplicity of design and control mechanisms.